To manually load most modern 35 mm cameras with film, a film leader is pulled out of a cassette, perforations located along at least one marginal edge of the leader are engaged with a metering sprocket and the free end of the leader is attached to a take-up spool located in a take-up chamber. The back cover of the camera is then closed and an initial film-winding operation is carried out by operating a winding lever and depressing a shutter release button several times until the first effective frame area of the film is in the proper position for exposure. Subsequent exposures are made by successively repeating the film winding and shutter releasing operations.
The metering sprocket is driven through one complete rotation by each actuation of the winding lever. During such rotation, the sprocket operates to recock the shutter and disable the shutter release button until the sprocket has completed its rotation, thus, preventing accidental exposure of the film prior to completion of its advancement.
One disadvantage of the aforementioned 35 mm loading mechanisms is that their complexity makes it difficult for many amateur camera operators to load 35 mm film properly. In some instances even though the camera operator believes that he has properly attached the free end of the leader to the take-up spool, the leader disengages from the spool after the camera back is closed. However, once the camera back is closed, improper film threading cannot readily be detected from outside the camera because the winding lever and shutter release button can operate in the usual way. The operator thus, under the delusion that the camera is working properly, learns of the film loading problem only when the film is developed.
U.S. Pat. No. 3,665,830 to Maeda discloses a 35 mm film transport mechanism having a first detection mechanism which detects whether or not the film is being wound on a take-up spool. The detection mechanism is associated with a release-button locking mechanism and a film-transport blocking mechanism so that until a predetermined amount of film has been wound on the take-up spool from a starting position, the film can be advanced without releasing the shutter. Upon detecting the winding of a predetermined amount of film, the film-transport blocking mechanism is enabled to block the film-winding operation and the release-button locking mechanism is freed from its retained position. Normal release-button locking and film-transport locking operations can then be effected. Also mounted in the camera is a second detection mechanism actuated upon detecting the loading of a film magazine. This detection mechanism is constructed such that when a film magazine is not loaded, the film-transport blocking mechanism and the release-button locking mechanism are allowed to function normally. Thus, by operating the winding lever, the shutter is freed to be released. Upon detecting the loading of a magazine, the second detection mechanism is released from the film-winding mechanism and the operation of the release-button locking means and the film-transport blocking means is controlled by the first detecting mechanism. The magazine detection mechanism allows test operation of the camera without film, which is desireable (1) during camera manufacture to check its accuracy, (2) in a store demonstration prior to sale to illustrate the camera`s features, and (3) for re-familiarization by the owner/operator after the camera has not been used for a prolonged period.
The Maeda film transport mechanism permits the camera operator to determine from outside the camera whether or not the film threading operation has been properly completed, while enabling test operation of the camera in the absence of film. However, this capability is achieved in Maeda by separate film-magazine and film-advancement detecting elements interconnected by a complex arrangement of cams, levers and springs.
It is well known to provide cartridge loaded cameras, (e.g. a camera using 110 or 126 size film cartridges), with mechanisms which permit test operation when there is no film in the camera, and which block the shutter operation when the camera is loaded with film until the film is advanced to a proper position for exposure. An example of such a mechanism is shown in U.S. Pat. No. 3,675,556 to Rigolini. Such mechanisms, however, can be inherently simpler than their 35 mm counterparts, because, in film cartridges, the film is permanently attached to a take-up spool and the film is provided with metering perforations associated with respective exposure areas at predetermined locations along the film strip.